public void SaveWorkbook(Workbook workbook)
{
    var dplWorkbook = workbook.ToDocument();

   // https://docs.telerik.com/devtools/document-processing/libraries/radspreadprocessing/formats-and-conversion/xlsx/xlsxformatprovider#export
    Telerik.Windows.Documents.Spreadsheet.FormatProviders.IWorkbookFormatProvider formatProvider = new Telerik.Windows.Documents.Spreadsheet.FormatProviders.OpenXml.Xlsx.XlsxFormatProvider();

    using (Stream output = new FileStream(_path, FileMode.Create))
    {
        formatProvider.Export(dplWorkbook, output);
    }
}